Harassed Galaxies

Most galaxies reside in dense swarms of up to a thousand, zooming around at some 600 miles a second. Astronomers first noticed some years ago that nearby galaxy clusters are populated with dim reddish elliptical galaxies. But the most distant and therefore oldest clusters are full of bright blue spirals, some of which are slightly irregular. The observations suggest that galaxies in clusters evolve from spirals to ellipticals over the course of about 5 billion years. Theorists have proposed various mechanisms to explain this--including collisions of galaxies--but none fully jibe with the data, says University of Washington astronomer George Lake.

Lake believes he and some colleagues may have solved the puzzle. They used a supercomputer to simulate the effect that living in a giant cluster of galaxies has on the shape and color of its members. Their computer model tracks a typical spiral galaxy as it orbits through a cluster consisting of a thousand other galaxies. Direct collisions aren’t needed to destroy a spiral, Lake’s team found. Rather, the gravitational pull of large galaxies passing nearby is enough to stretch a spiral like a piece of taffy.

The result of such galactic harassment after a few billion years is that a spiral galaxy, with its stars in orderly arrays of spiral arms, is distorted into a less organized, elliptical blob. At the same time, the galaxy gets redder, simply because that’s what most of its constituent stars do as they get old and burned out. A comparison of the model-generated shapes with images from the Hubble Space Telescope shows remarkable similarities between artificial and actual galaxies.

Lake says the model may also provide the answer to the recently discovered mystery of naked quasars. Quasars are incredibly bright powerhouses of radiation that are believed to be fueled by gas falling into a massive black hole at the core of a galaxy. But naked quasars reside in galaxies that seem to contain far too little gas. Such galaxies may simply be harassed spirals on their way to becoming ellipticals, says Lake. While harassment tears away some of a galaxy’s free gas and dust, he explains, it just slows the orbit of the rest. That material may fall in a concentrated stream toward the galactic core--and it could ignite a quasar.